installation instructions not working

[Rykath]

I've encountered a few problems when trying to install SIMPLE:

1. GSL is not mentioned as a requirement, installing libgsl-dev on ubuntu helped.
2. the pyproject.toml is broken: it should have tool.setuptools.package-data instead of project.package-data

[krystophny]

Root cause of the GSL part: SIMPLE links GSL transitively via libneo's collision_freqs (PUBLIC GSL::gsl), which is the undocumented build requirement. GSL is GPL, so this is also a license conflict for the MIT codes. Tracked and to be fixed in itpplasma/libneo#285 (drop GSL, use intrinsic gamma() + in-tree incomplete gamma). The pyproject.toml part of this issue was fixed in #322.